1. Field of the Invention
The present invention relates to a process for preparing an insulating film for semiconductor memory devices. The insulating film according to this invention is suitably used for a capacitor of a highly integrated memory device such as a DRAM.
2. Description of the Prior Art
As semiconductor memory devices are being more highly integrated, thinner insulating films for such devices are required. It has been a conventional practice to use a thermally oxidized SiO.sub.2 film as the insulating film. However, when the thickness of the SiO.sub.2 film is below 100 .ANG., which is required for high integration, the film is heterogeneous in thickness thereby lowering the reliability thereof.
Therefore, a laminated structure is now given to an insulating film having a thickness below 100 .ANG. so that the reliability can be improved. Specifically, as will be described below, a silicon nitride film prepared by CVD is used as a part of the laminated structure.
The silicon nitride film, which has a good homogeneity in thickness, has many pinholes therein and allows the Poule-Frenkel type conduction, with the result that the film alone is inferior to a SiO.sub.2 film in dielectric strength. Thus, there has been proposed to constitute an insulating film for memory device with the laminated structure of (top) silicon oxide/silicon nitride (bottom) which is obtained by thermally oxidizing to 10-30 .ANG. in depth the top of the silicon nitride film, or the structure of (top) silicon oxide/silicon nitride/silicon oxide (bottom) which is provided with a thermally oxidized SiO.sub.2 film under the former structure U.S. Pat. No. 4,240,092; "Inter-Poly SiO.sub.2 /Si.sub.3 N.sub.4 Capacitor Films 5nm Thick for Deep Submicron LSIs" Extended Abstracts of the 20th (1988 International) Conference on Solid State Devices and Materials, Tokyo, 1988, pp. 173-176; "TDDB Measurement of SiO.sub.2 Gate and SiO.sub.2 /Si.sub.3 N.sub.4 /SiO.sub.2 Gate Structure" Extended Abstracts of the 17th Conference on Solid State Devices and Materials, Tokyo, 1985, pp. 267-270]. The silicon oxide/silicon nitride/silicon oxide laminated structure is actually used for 1M-bit DRAMs or 4M-bit DRAMs.
Laminated structure of an insulating film for memory devices improves the reliability thereof, while decreasing the capacitance thereof, because the structure provides a serial connection of capacitor elements. Thus, the silicon oxide/silicon nitride laminated structure resulting from the oxidation of the top of the silicon nitride film improves the reliability of dielectric strength of the insulating film, but causes an unavoidable decrease in the capacitance. Such a decrease in the capacitance may induce malfunction of the memory devices, thereby lowering the reliability thereof.